This invention relates to a starter system for an engine, such as an internal combustion engine or gas turbine jet engine. More particularly, the starter system includes a starter motor which functions as an alternating current (AC) generator once the engine has started.
The number of electrically driven accessories and actuators on motor vehicles and aircraft has been increasing. Electrical control of mirror positioning, electrical extension and retraction of antennas, and electrical latching of trunk lids are features which have been seen on some automobile designs within the last few years. Relatively high power levels have been required by some electrical features such as electrical positioning of the driver's seat, electrical heating of the driver's seat, electrical power steering, and future electrically-driven air conditioner compressors. Improved efficiency, reliability, weight reduction and diagnostic capability are design goals for motor vehicles and aircraft which increasingly use computer controls that facilitate such improvements. The increasing use of electrically driven accessories and actuators has resulted in greater requirements for on-board generation of electrical power.
A conventional internal combustion engine or gas turbine jet engine contains a direct current (DC) motor, which functions as a starter, and a separate DC generator or alternator. (As used herein, "generator" may signify either a DC or AC generator, an AC generator often being called an alternator.) The starter motor is typically powered from a battery through a high current, solenoid-operated switch. In order to provide the high torque necessary to start the engine turning, torque multiplication is achieved through a high ratio (typically 10:1 to 15:1) gear train. During automotive or other motor vehicle engine cranking, a small gear on the starter engages, via a Bendix drive, with a large gear mounted on the circumference of the engine flywheel. While the engine is running, the starter is mechanically disengaged from the flywheel and provides no useful function. A DC generator or an alternator (combined with an uncontrolled diode rectifier bridge and regulator) maintains the battery charge, which is depleted during starting of the engine, and supplies electrical power for on-board accessories while the engine is running.
Typically, motor vehicle alternators are driven from the engine crankshaft by a belt and pulley arrangement. The alternator operates at higher speed than the crankshaft to provide adequate voltage to maintain battery charge even when the engine runs at low idle speed. As engine speed increases, a regulator reduces the alternator field to maintain proper voltage for charging the battery. Conventional alternator efficiency is quite low, having a maximum of 55% to 60% which decreases to about 38% to 45% as the alternator speed and load increase to the maximum speed and rated load. Additional system losses, especially at high speed, result from alternator drive belt/pulley losses, alternator fan losses, and machine losses while operating at high frequencies.
The conventional system using a DC starter motor and alternator is disadvantageous for a number of reasons. The starter motor is not used except when the engine is being cranked/started, whereas the alternator is used only while the engine is running. The component which is not being used at a particular time simply increases the weight, size, and number of hardware components required to be carried by the vehicle. The requirement that the starter motor engage the flywheel during engine cranking/starting and disengage during engine operation introduces additional complexity into the system. Electric machine design, simplistic controls, and fan and belt/pulley losses of conventional starter and generator systems keep the efficiency relatively low. Additionally, the belt is subject to breakage or slip. Further, some prior starting and generating systems for aircraft or motor vehicles may use commutators, brushes, slip-rings, or other components which readily wear out.